Robot Gliders See How Antarctic Ice Melts From Below

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Scientists suspect Antarctica's shrinking glaciers are melting
from the bottom up, and a fleet of robot ocean gliders may help
explain why.

Beneath the icy Weddell Sea in West
Antarctica, the gliders discovered turbulent warm currents
near ice shelves, the huge floating platforms where continental
glaciers extend icy tongues into the sea. The swirling eddies
carry pulses of warm water to the shallow depths underneath the
ice, scientists report today (Nov. 10) in the journal Nature
Geoscience.

"What we're looking at is delivery of heat right up to the ice
shelf, where the ocean touches up against the ice," said lead
study author Andrew Thompson, a physical oceanographer at
Caltech. "It's almost like a blob of warm water, a little ocean
storm." [ Album:
Stunning Photos of Antarctic Ice ]

Previous work already pointed to warm water — rather than hotter
air temperatures — as
the reason for Antarctica's retreating ice shelves. (The
disappearing ice is part of the continental ice sheet, not the
sea ice that freezes and melts each year.) But to confirm these
suspicions, the researchers needed to get under the ice to see
how the process works.

In 2012, Thompson and colleagues from the University of East
Anglia, in the United Kingdom, used remotely operated gliders to
probe the ocean conditions near ice shelves in the Weddell Sea.
The gliders rise and sink without propellers, relying instead on
a battery-driven pump that changes their buoyancy via a
fluid-filled bladder. Every few hours, the six-foot-long (1.8
meters) glider surfaces and uploads its data via a satellite
phone network. The gliders collected temperature and salinity
data for two months, exploring the upper 0.6 miles (1 kilometer)
of the ocean.

When the gliders hit an eddy, the sleek yellow robots were often
caught up in the powerful vortices. "You could almost know by
where it came up that it had hit this anomalous region," Thompson
told Live Science. "The glider would go down and end up in a
quite different place."

The findings are the first to explain how warm water rises from
deeper levels to reach the floating ice shelves. The results
suggest the stormlike currents bring up pulses of warm water,
which flow under the ice at irregular intervals. Now, researchers
need to find out what happens when this heat reaches the
grounding line, the spot where glaciers transfer their weight
from the continent to the ocean. This is where most of the
melting takes place, Thompson said.

"What we're seeing from the gliders is that it's not a steady
circulation in and out," Thompson said. "This is really the first
step of understanding of what heat goes in, and how efficient
that heat is in melting the ice shelves."

Alternating layers of cold and warm water surround Antarctica,
and it only takes a few degrees of difference to dissolve a
glacier. The warmer water is typically in the middle layer of the
ocean. It arrives from the north, delivered on a giant current
called the global
conveyor belt. Colder water lies on the surface, often formed
when cold wind blows over the ocean and sea ice freezes up.
Dense, cold water is also on the ocean bottom.